Many reactions carried out in chemical technology proceed under the influence of radicals, the formation of which can proceed in various ways.
Thus, chlorination of the lateral chain of toluene --a well-known substitution reaction --may proceed under the influence of light, as well as of a chemical radical initiator, such as radicals formed from peroxide. Another example is the polymerization of styrene, which can be carried out under the influence of heat, as well as with the aid of a peroxide. In general, the higher reaction rate obtainable therewith is an advantage of the use of a chemical radical initiator. Among other things, a disadvantage may be by-products liberated during its degradation. Although the quantity of radical initiator used is in general very small, the concentration of formed by-products is frequently still so high, that the presence thereof in the end product may even be harmful to its quality. The latter may also occur in the case of polymers formed under the influence of radicals; the removal of by-products from these is frequently not possible.
Use of a purely thermal polycondensation is afflicted with the drawback of a long duration, which is necessary to obtain an adequately high degree of polymerization. In order to meet this drawback, radical initiators have indeed been proposed, which being to develop their activity only at elevated temperatures, e.g., between 150.degree. and 250.degree. C. Below this temperature, these initiators are practically stable, so that they are not used up during the "main polymerization" under the influence of heat and therefore may be added to the reaction mixture already at the beginning of polymerization. An important advantage of this characteristic is the fact that only very little of these initiators needs to be added to the mixture to be polymerized, since they only have the function to accelerate the progress of "afterpolymerization," at the beginning of which mostly not more than about 5% of the initial quantity of monomer is present.
Examples of such radical formers, to which belong, among others, certain 1,2-diaryl- 1,1,2,2,-tetraalkylethane compounds, are mentioned in British Pat. No. 864,675. In this patent, preference is given to 1,2-diaryl-1,1,2,2-tetraalkylethane compounds in which at least one alkyl group with more than one C atoms is bonded to each of the two C atoms of the ethane group. As an example, mention is made of (.alpha.-methyl .alpha.-ethyl- .beta.-methyl-.beta.ethyl)-diphenylethane (=3,4, -diphenyl-3,4 -dimethylhexane). The last-mentioned type of compound appears to be very suitable for the application in question.
However, a drawback of these compounds is their high price. The reason is, that their preparation starts with the relatively high expensive secondary butylbenzene, which is dimerized in a manner analogous to cumene. The dimerization product of the last-mentioned substance is dicumyl (or dicumene), which can be considered the simplest representative of the mentioned tetraalkylethane compounds. The price of cumene is relatively low, but the initiator activity of the dimer appears to be insufficient for practical applications.